The present invention relates to a method and apparatus for providing an indication of the composition of an examined fluid. The invention is particularly useful in a heat pump for indicating the physical, or phase, composition of the working fluid (i.e., the relative proportions of the working fluid in the liquid and vapor phases) at one or more stages of the heat pump cycle. Another application is for indicating the liquid level in a vaporizer. The invention is therefore described below particularly with respect to these two applications, but it will be appreciated that the invention is useful in many other applications.
Heat pumps are widely used for heating and/or cooling purposes. They include a phase-changing working fluid driven through a closed loop including a compressor, a condenser, an expansion valve, and an evaporator. The proper and efficient operation of such a heat pump depends to a considerable degree on the ability to control the physical composition of the working fluid (i.e., the relative proportions in the liquid and vapor phases) at the different stages of the closed loop cycle, particularly at the inlet to the evaporator, and at the outlet from the evaporator. Thus, if the working fluid is not completely in the liquid phase at the inlet to the evaporator, and not completely in the vapor phase at the outlet from the evaporator, the overall efficiency of the heat pump cycle could be significantly lowered; moreover, the loss of working fluid by leakage could go undetected.
Many Vapor Cycle Systems (VCS) use pressure sensors to measure the pressure at the various stages of the cycle, but pressure sensors tend to provide false alarms at cold conditions below 0xc2x0 C., or at low heat load conditions. Sometimes, in order to overcome false alarms, the system may include a temperature valve that disconnects the low pressure information at low temperatures, to thereby avoid a false alarm. This solution is frequently applied to systems used in avionics, but such a solution does not solve the problem in case of a real leak.
There are many other areas where it is highly desirable to be able to provide an indication of the composition or phase content of an examined fluid in a quick and simple manner. One such other application is for indicating the liquid level in a vaporizer outlet. Other possible applications would be for identifying liquid flow in an oil system, water flow in an irrigation system, liquid coolant flow in a cooling system, and other liquid, gas or vapor flow systems.
An important object of the present invention is to provide a novel method, and also a novel apparatus, for indicating the composition of a fluid in a quick and facile manner. Another object of the invention is to provide a novel method and apparatus for indicating the relative proportions of a working fluid that are in the liquid and vapor phases in one or more stages of a heat pump. A further object is to provide a novel method and apparatus for indicating the liquid level in a vaporizer.
According to one aspect of the present invention, the invention relates to a method of providing an indication of the composition of an examined fluid, comprising:
immersing an electrical resistor within the examined fluid; applying electrical current through the electrical resistor to heat it to a temperature above that of the examined fluid; controlling the electrical current applied through the electrical resistor to maintain the electrical resistor substantially at a predetermined temperature while the electrical resistor is immersed in the fluid; measuring the electrical current applied through the electrical resistor to maintain it substantially at the predetermined temperature, to provide a measurement of the rate of heat dissipation from the electrical resistor via the examined fluid and, thereby, a measurement of the thermal conductivity of the examined fluid; and comparing the measured thermal conductivity of the examined fluid with the known thermal conductivities of different fluid compositions to provide an indication of the composition of the examined fluid.
According to an important preferred feature of the present invention, the electrical resistor is a positive temperature coefficient (PTC) thermistor. Such thermistors can be constructed by known doping and sintering processes to have a maximum predetermined temperature irrespective of the power supplied to the thermistor, thereby obviating the need for additional controls or temperature sensors when used as the electrical resistor immersed in the fluid to be examined in accordance with the present invention. Many other important advantages in the use of such thermistors are described below.
The invention is particularly useful where the fluid is examined for its physical composition, e.g., to determined the relative proportions thereof in the liquid and gaseous phases. One particular application of the invention is in the examination of a phase-changing working fluid in one or more stages of a heat pump for indicating the proportions of the working fluid in the liquid and vapor phases at the respective stage of the heat pump cycle. The results of the examination of the working fluid may be used for optimizing the heat pump cycle, as well as for detecting leakage of working fluid, etc.
According to a second described preferred embodiment, the fluid is a liquid in a vaporizer inlet, and is examined at one or more levels therein to indicate the vapor content in the vaporizer outlet.
While the invention is particularly useful, and is therefore described below, for indicating the physical composition of a fluid (e.g., the relative proportions in the liquid and vapor states), the invention may also be used for determining the chemical composition of a fluid, (e.g., a mixture of different fluids having different thermal properties) in which case the measured thermal property of the examined fluid would be compared with the known thermal property of fluids of different compositions to provide an indication of the chemical composition of the examined fluid.
When the invention is used for indicating either the physical composition or the chemical composition of a fluid, the fluid should of course be non-flowing, or flowing at a known velocity, so that the measured thermal conductivity can be compared to the known thermal property of other fluids under the same velocity conditions. Once the fluid has been examined to provide an indication of its composition, it may then be examined while flowing at an unknown velocity also to indicate its velocity.
According to another aspect, the invention also relates to apparatus for providing an indication of the composition of an examined fluid, comprising: an electrical resistor having a mounting device for mounting the electrical resistor immersed in the fluid to be examined; a power supply for supplying electrical current to the electrical resistor to heat it to a temperature above that of the examined fluid; and a processor for: (a) controlling the heating current applied to the electrical resistor to maintain the electrical resistor substantially at a predetermined temperature; (b) measuring the electrical current applied to the electrical resistor to maintain it substantially at the predetermined temperature; and (c) producing an output, based on the measured electrical current, indicating the composition of the examined fluid. Particularly good results are obtained, as indicated above and as described more particularly below, when the electrical resistor is a positive temperature coefficient (PTC) thermistor.
Further features and advantages of the invention will be apparent from the description below.